A cellular wireless network may include a number of base stations that each radiate to provide coverage in which to serve wireless communication devices (WCDs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wireless equipped communication devices. In turn, each base station may sit as a node on a core access network that includes entities such as a network controller and a gateway system that provides connectivity with an external transport network such as the Internet. With this arrangement, a WCD within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other WCDs.
In an example arrangement, the gateway system may include one or more packet data network gateways (PGWs) or similar components that provide connectivity with a packet-switched network so as to support general packet-data communications, such as general web browsing, file transfer, and the like, and/or packet-based real-time media communications such as voice over Internet Protocol (VoIP) and streaming media for instance.
A representative PGW may sit as a node on a wireless service provider's private packet-switched network and may thus provide connectivity with various application servers and other entities on that private network, and with other such entities accessible through a connection between the service provider's network and one or more other networks such as the public Internet. By way of example, such a PGW may provide connectivity with an Internet Multimedia Subsystem (IMS) platform or other session server that supports VoIP calling and/or other such media services.
When a WCD first enters into the coverage of a cellular wireless network, the WCD engages in a process of registering or “attaching” with the network, which may trigger setup of various communication channels for the WCD and/or reservation of various communication resources for the WCD. For instance, upon first detecting coverage of a base station, the WCD may transmit an attach request message to the base station, which the base station may forward to the network controller, such as a mobility management entity (MME). Upon authenticating and authorizing the WCD, the network controller may then engage in further signaling with the base station and with a serving gateway (SGW), which may in turn engage in signaling with a PGW, ultimately resulting in setup of one or more bearer connections or “bearers” each extending, via the base station, between the WCD and the PGW, through which the WCD can then engage in packet-data communication via the PGW. In addition, the base station may maintain a context record indicting the bearers(s) currently established for the WCD.
Each bearer established for a WCD may define a logical communication tunnel that includes a radio bearer portion extending between the WCD and the base station, and an access bearer portion extending between the base station and the PGW via the SGW. Further, each bearer may have a quality of service (QoS) level that defines for the bearer one or more QoS parameters that dictate how the packet data transmission along the bearer is handled by the network. For instance, from the base station's perspective, the QoS parameters may establish for the bearer a priority level according to which the base station should schedule air interface communication of data on the bearer. Such QoS parameters may include, for example, a packet delay budget (i.e., a maximum tolerated time that a packet may be delayed between the WCD and the PGW), a maximum tolerated rate of packet loss (also known as “error tolerance”), and/or a minimum tolerated bit rate.
Further, a bearer's QoS level may be represented in practice by a QoS class identifier (QCI) value. For example, LTE defines QCI values 1 through 9, with QCI values 1-4 providing for guaranteed bit rate (GBR) communications, and with QCI values 5-9 providing for best-efforts (non-GBR) communications. The QCI value of a bearer may also associate the bearer with a particular type of communication. For example, in LTE, a QCI 1 bearer is associated specifically with VoIP-call communication, and a QCI 9 bearer is associated specifically with general Internet traffic such as e-mail, chat, file transfer, and the like.
Each QCI level may thus correspond with data communication that has a particular priority level and particular QoS parameters. For example, a QCI 1 bearer may be for highest-priority data communications and could have a relatively low packet delay budget, a relatively low error tolerance, and/or a relatively high minimum bit rate. Further, a QCI 5 bearer may be for mid-level-priority data communications and could have a mid-range packet delay budget, a mid-range error tolerance, and/or a mid-range minimum bit rate. And a QCI 9 bearer may be for lowest-priority data communications and could have a relatively high packet delay budget, a relatively high error tolerance, and/or a relatively low minimum bit rate.
In practice, the network may establish for a WCD multiple bearers, each having different QoS levels and different QoS parameters. In particular, the network may initially establish for a WCD one or more default bearers to enable the WCD to engage in certain basic communications. By way of example, the network may initially establish for the WCD a default mid-level-QoS IMS signaling bearer (e.g., a QCI 5 bearer), which the WCD could use to engage in Session Initiation Protocol (SIP) signaling with the IMS to facilitate setup of a packet-based real-time media session, such as a Voice over LTE (VoLTE) call. Then, when the WCD engages in SIP signaling over the default IMS signaling bearer, the network may set up for the WCD a dedicated high-QoS IMS bearer (e.g., a QCI 1 bearer) for carrying VoLTE call traffic (namely, packets carrying digitized speech to or from the WCD in a voice call). Thereafter, while the WCD is engaged in the VoLTE call, additional SIP signaling for supporting the VoLTE call may be carried on the default IMS signaling bearer.